The aim of this work was to learn more about the chemical mechanisms by which nitric oxide (NO) is created an destroyed in combustion. Premixed, laminar flames of H<SUB>2</SUB> + O<SUB>2</SUB> + N<SUB>2</SUB> and CH<SUB>4</SUB> + O<SUB>2</SUB> + N<SUB>2</SUB>, burning at atmospheric pressure with final temperatures between 1840 and 2520 K were studied. Temperature was measured along the axes of these flames using coherent anti-Stokes Raman spectroscopy (CARS). A procedure was developed for measuring the concentration of OH along the axis of these flames from a two-dimensional image of laser induced fluorescence (LIF). This method compensated for variations in the width and intensity of the laser sheet used to excite the molecules, required only one calibration and was applicable to either fuel-lean or fuel-rich flames burning unshielded from the entrainment of air. Concentrations of NO<SUB>x</SUB> (here, NO<SUB>x</SUB> = NO + NO<SUB>2</SUB> + HNO, but [NO]/[NO<SUB>x</SUB>]>0.98) were measured along the axes of the flames. Whereas the Zel'dovich mechanism was sufficient to explain the formation of NO<SUB>x</SUB> in the burnt gases of fuel-lean flames, NO<SUB>x</SUB> was produced faster than predicted by the Zel'dovich mechanism alone in fuel-rich flames with final temperatures below 2100 K. Including the production of NO <I>via </I>NNH, in N<SUB>2</SUB> + H = N<SUB>2</SUB>H, followed by N<SUB>2</SUB>H + O → NO + NH, gave a reasonable fit to the measurements. Predictions of [NO<SUB>x</SUB>] obtained using a reaction scheme for combustion devised by Miller & Bowman (1989, <I>Prog. Energy Combust. Sci., </I>15, 287) were below the measured [NO<SUB>x</SUB>] in the cooler (T < 2050 K), fuel-rich flames of hydrogen, because this scheme does not include formation of NO <I>via</I> NNH. Miller & Bowman's (1989) mechanism was also found to underpredict [OH] in fuel-rich and [NO<SUB>x</SUB>] in fuel-lean and fuel-rich flames of methane. The concentration of NO<SUB>x</SUB> was also measured in flames of H<SUB>2</SUB> + O<SUB>2</SUB> + Ar, with traces of NO or NH<SUB>3</SUB> added to the burner supplies. For flames seeded with NO, some of the additive (up to 60%) was destroyed in the reaction zone. When either NO or NH<SUB>3</SUB> was added, [NO<SUB>x</SUB>] remained constant along the burnt gases of a fuel-lean flame but increased downstream of the reaction zone of a fuel-rich flame; this rapid loss and subsequent regeneration of NO<SUB>x</SUB> in a fuel-rich flame doped with NO was not predicted by several models of combustion chemistry.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:604855 |
| Date | January 1997 |
| Creators | Hutchinson, E. M. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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